ctures of the Asphaltic Fractions b arious llnstrumental Methods John P. Dickie and Teh Fu Yen Meilon Institute of Carnegie-Mellon University, Pittsburgh, Pa.
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The asphaltene fractions from seven different native crude petroleums, the resin fractions from four of these crudes, one gilsonite asphaltene, and one asphaltene from a heavy asphaltic oil were investigated by a variety of physical methods. The molecular weights measured by each of these methods were used to form a hypothesis about the macrostructure of the asphaltic material. t h e divergent results of molecular weight measurement are explained by an asphalt “moiecule” composed of individual sheets, which associate to form unit cells and larger associated micelles. Resins are similar to the corresponding asphaltene fractions in size but differ in the extent of condensation. A model for the macrostructure of asphaltics based on the present observations arid previous data is presented.
THEMOLECULAR WEIGHTS of asphaltic materials have been the subject of several studies which have yielded divergent results depending upon the method employed (Table I). Published data for the molecular weight of petroleum asphaltenes range from approximately 1000 to 500,000. Specifically, data obtained from solution viscosity, ebullioscopic, and crysscopic methods are low, whereas those from ultracentrifuge and electron microscope studies are high. The asphaltene and resin fractions of petroleum have been pictured as repeating units of similar composition ( I ) , with the difference, apart from solubility, being mainly in the aromaticity of the materials. The fractions are thought to consist of a twodimensioned fabric of condensed aromatic rings, short aliphatic chains, and naphthenic ring structures combined in a three-dimensional network (2). It is apparent from this picture that the concept of molecular weight may not be applicable unless the part of the molecule which is being measured is well defined. For example, one molecular weight which might be determined is the unit cell (particle) as shown in Figure 1, C. This particle has been studied by x-ray (3) and visualized by use of the electronic microscope (2). Another molecular weight would be that of one of the individual layers of this unit cell (Figure 1, J). Perhaps it would be correct to call this the ultimate or minimum molecular weight. Still another molecular weight which might be measured is that derived from the association of several of these unit cells into larger nuclei, held together by intermolecular forces (Figure 1, D). It is thought that this association may contribute to larger molecular weights measured under certain conditions. In fact, these associated unit particles or micelles have already been observed in electron micrographs and ultracentrifugation patterns (2,4,5).
(1) T. F. Yen and J. G. Erdman, ACS, Div. Petroleum Chem., Preprints, 7 (3), 99 (1962). (2) J. P. Dickie, and T. F.Yen, Zbid.,11 (3), 39 (1966). (3) T. F. Yen, J. G. Erdman, and S. S. Pollack, ANAL.CHEM., 33, 1587 (1961).
(4) R. S . Winniford, J . Inst. Petrol., 49,215 (1963). ( 5 ) P. A. Witherspoon, R. I. No. 206, Illinois State Geological Survey, 3 958.
Figure 1. ~ a c r o s ~ of~ asphaltics ~ ~ ~ ~ r e A.
Crystallite
C. Particle E. Weak link G . Intracluster
Z. Resin K. Petroporphyrin
B.
Chain ~
u
~
~
D. Micelle F. Gap and hole H . Intercluster J. Single layer L. Metal
The purpose of the present work was to examine molecular weights determined by a variety of methods and to attempt to correlate these values with current theories of structure. The ultimate objective is a better understanding of the framework or the macrostructure of the asphaltic material. EXPERIMENTAL
X-Ray Diffraction and Scattering (High Angle and Low Angle). The high angle x-ray studies were carried out as previously described (3) employing either of two instruments: the first, a Norelco x-ray diffractometer equipped with a copper K-alpha radiation source and a Geiger tube detector; the other, a Picker Horizontal x-ray diffractometer with a copper K-alpha radiation source, a gas proportional detector, and pulse height discriminator. Intensities were measured over a range of 28 = 8-100” and parameters calculated as describe earlier (6). A summary of the results obtained by calculation of molecular weights from x-ray measurements may be found in Table 11. From a knowledge of C A and CA’ (7) and the value of the aromaticity ( f a ) as determined from x-ray messurement, it is possible to calculate a sheet molecular weight for petroleum asphaltic materials. As previous investigations ( I ) have shown that the aromatic systems in asphaltene fall somewhere between the circular condensed peri type (6) T. F. Yen and J. C. Erdman, in “Encyclopedia of X-rays and Gamma Rays,” G. L. Clark, Ed., Reinhold, New Yo&, 1963, p. 65. (7) T. F. Yen and J. P. Dickie, ACS, Div. Petroleum @hem,,Preprints, 11 (3), 49 (1966). VOb. 39, NO. 14, DECEMBER 1967
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Table I. Various Methods for the Determination of Molecular Weight of Asphaltics Range Investigator 2,500-5,000 R. L. Griffin, W. C. Simpson, and F.K. Miles, ACS, Div. of Petroleum Chem., Preprints, 3, No. 2,A13 (1958). A.N.Sakhanov and N. A.Vassiliev, C.A., 28, 298 (1934). 5,000-6,OOO Cryoscopic, benzene M.Katz, Can. J. Research, 10, 435 (1934). 1!700 E.S. Hilman and B. Barnett, Proc. Am. Soc. Testing Materials, 37, 558 (1937). Cryoscopic, naphthalene Cryoscopic, phenanthrene 2,500 M.L. Boyd and D. S. Montgomery, Fuel, 41, 335 (1962). 900-4,000 K.A. Fischer and A.Schram, 5th World Petroleum Congress, New York, Sec. Viscosity V, Paper 20 (1959). 6. W. Eckert and B. Weetman, I d . Eng. @hem.39, 1512 (1947). 20,000-80,000 J. W. A. Labout and J. P. H. Pfeiffer, “Properties of Asphaltic Bitumen,” Osmotic pressure Elsevier, p. 36, 1950. W. M. Zarrella and W. E. Hanson, Geological SOC.Am. meeting, Denver, 1960. 1 3 ,OOCL46 ,009 P. A. Witherspoon, Ill. State Geological Survey, R. I., No. 206 (1958). Ultracentrifuge (r = 15.9-24.7A) B. R.Ray, P. A. Witherspoon, and R. E. Grim, J. Phys. Chem., 61,1296(1957). 50,000-2,500,000 R. S. Winniford, J. Inst. Petroleum, 49, 215 (1963). Ultracentrifuge D.L. Katz aqd K. E. Beu, Ind. Eng. Chem., 37, 195 (1945).
